1. Field of the Invention
The present invention relates to high pressure air guns used in marine seismic exploration. More particularly, the present invention relates to a method and apparatus for substantially suppressing the undesired recoil derivative from the explosive discharge of high pressure gas from air guns while still providing for useful optimized pressure pulses.
2. Description of the Prior Art
In marine seismic exploration, a series of strong acoustical pulses or waves are generated in the water. These pulses rapidly pass through the water and the geological formation and are reflected at the surface for recording and interpretation. Though there exist a number of sources used in seismic exploration, the more popular seismic source is the air gun produced by companies such as Bolt, Inc. or Haliburton Geophysical Services, Inc. and improved seismic sources produced by Seismic Systems, Inc. such as those described in U.S. Pat. No. 5,018,115.
The air gun generally consists of an elongated annular housing defining a cylindrical chamber which is provided with a sealable port at one end and a shuttle valve which is slidably disposed along the chamber axis within or around said housing. When the shuttle is maintained in a first or "closed" position air is pressurized within the chamber by a sealable port defined thereby. When the shuttle is moved to a second or "open" position, the pressurized air is suddenly and explosively discharged into the water through one or more discharge portal(s) formed opposite the sealable port in the housing, which portal(s) communicate with the surrounding water.
When the pressurized gas is first released from the sealable port and is still confined within the chamber, the gas acquires a velocity parallel to the chamber walls, or a direction parallel to the axis defined by the chamber itself. In the vicinity of the sealable port, the cross-sectional flow area is at a minimum and the gas typically reaches a velocity approximating the speed of sound, or approximately 340 meters per second at standard temperature. This portion of the chamber is commonly referred to as the throat. At the throat, the gas molecules, having acquired a velocity close to that of sound and a flow direction paralleling the axis of chamber, will tend to follow a flow line as close as possible to chamber axis. The precise flow line of the pressurized gas depends on the geometry of the chamber, the geometry of the sealable port, obstacles encountered on the natural flow path of the gas molecules as well as the interaction between parallel flow lines of the gas molecules.
When the gas molecules reach the end of the housing opposite the sealable port, they vary from their flow direction parallel the chamber axis as they impact the distal end of the housing (or of the shuttle valve depending on the architecture of the air gun) and are forced outwardly through the discharge portal(s). While it might appear as though the gas molecules would then adopt a direction perpendicular to the axis of the chamber, the gas molecules instead adopt flow lines which on average define an acute angle of some 45.degree.-60.degree. with the axis defined by the chamber. The momentum carried by the mass of the gas flowing into the water with the aforedescribed high velocity creates a directional vector which tends to move the air gun axially in a direction opposite the vectored flow lines of the gas, thereby inducing a recoil.
In operation, the air gun is typically towed behind a specially equipped surface vessel whereupon the gun is actuated at selected, closely spaced intervals. When used in such a fashion, the air gun is tethered to the surface vessel by a heavy gauge cable or harness to which is coupled electrical cable to actuate the air gun, and high pressure air lines to supply pressurized gas to the gun. The air lines are coupled to large compressors on-board the marine vessel.
The aforedescribed axial movement of the housing induced by the discharge of gases at repetition frequencies in the nature of every ten seconds, frequently causes considerable wear and fatigue on the harness, electrical cables and high pressure air lines. For obvious reasons, this wear is undesirable and necessitates frequent repair and replacement of these components with incident costs associated with down-time of the air gun and support equipment.
A variety of apparatus have been proposed to counter the aforedescribed and undesired effects resultant from the recoil of the air gun. One such solution is that proposed in European Patent Application No. 0355954. In this proposed design, the air gun is attached to a solid block or frame of substantial mass in an effort to minimize the recoil of the air gun during operation. This design, however, is undesirable since the frame is heavy, cumbersome and quite expensive.
In seismic exploration, it is desired to create a large primary pulse, which is useful is seismic exploration, while reducing secondary, pulses which distort the acoustic signature of the primary pulse, and therefore reduce the quality of the portrayal of the marine geological formation. As is well known in the art, the primary pulse is created by the initial discharge of pressurized gas. This gas quickly expands to form a bubble. The collapse and reexpansion of this bubble creates the secondary pulse. The elimination or reduction of the secondary pulse has been the subject of considerable study and research.
To maximize the signature of the primary acoustic pulse, air guns are frequently used in clusters. In the cluster, two or more air guns are coupled together in sufficient proximity that the high pressure gas discharges from each gun coalesce and thus achieve a greater primary pulse then that achievable using a single gun. In a "clustered" mode, however, the undesired recoil effects described above are exacerbated. This is discussed in U.K. Patent No. 2176605A as well as in an article entitled "Air Gun Interdependency Pursuit of the Spectral Limits" by John C. Write & Dewey R. Young as published in the SEG Annual Meeting in 1988.
Further, in the air gun cluster, the spacing between the air guns is highly dependent upon the depth at which the cluster is fired and the air pressure supplied to each air gun. When the air guns are situated in a block or frame in an effort to minimize their recoil, a change in the firing depth or air pressure of the cluster necessitates a corrected spacing of the guns in the frame. This respacing is both cumbersome and time consuming, and, consequently, results in enhanced costs.